1. Field of the Invention
The present invention relates to a developing unit for developing a latent image formed on a photosensitive member in an image forming apparatus use in an electrostatic photographic process, and more particularly to a one component developing system which visualizes a latent image of a photosensitive member by bringing a thin layer of developer into contact with the photosensitive member.
2. Description of the Related Art
In an image forming apparatus, for example, an electrostatic copy machine, a latent image, which is obtained by applying illumination to an image of a document to correspond to reflected light from the document, is formed on a photosensitive member, which is charged to a predetermined polarity in advance. The latent image formed on the photosensitive member is developed with toner supplied from a developing unit to serve as a toner image. Thereafter, the toner image is transferred to a sheet paper provided on a surface of the photosensitive member to serve as a copy image. The toner image transferred onto the sheet paper is separated from the photosensitive member together with the sheet paper, and fixed to the sheet paper by a fixing unit. At this time, toner is melted by heat of a heater roller and is pressurized by press of the heater roller of the fixing unit. Melted toner is introduced between the fibers of the sheet paper and toner, so that the sheet paper and toner are firmly adhered to each other to be discharged as a copying output.
In recent years, as compared with the two-component developing system, the developing unit of one-component developing system has been widely used in a printer. The reason can be considered as follows.
More specifically, mixture with toner and carrier and stirring, and management of toner concentration in developer, i.e., mixing agent of carrier and toner are not needed. Replacement of developer with new one is not needed, and a size of the developing unit can be miniaturized.
As a developing unit of one component developing system, there are known a developing unit using magnetic toner and a developing unit using non-magnetic toner. However, a uniform thin layer of toner is formed on the surface (outer peripheral surface) of a developing roller by a toner restricting member regardless of the, kinds of toner.
One reason why the thin layer of toner must be formed on the developing roller is that toner supplied to the photosensitive member from the developing unit is transferred without fail. In other words, magnetic force of one component magnetic toner is smaller than that of two component developer (the size of the well-known magnetic brush is small), and is transferred by only magnetic constraint of toner. Also, one component non-magnetic toner cannot use a well-known magnetic brush, and is transferred by only electrical charge (optical antipode force), which toner has, and/or Coulomb's force by frictional electrification. Due to this, the distance between the developing roller and the photosensitive member must be narrowed as compared with the case of two component developing system.
The other reason why the thin layer of toner must be formed on the developing roller is that toner must be uniformly charged. In order to form the thin layer of toner, the following methods can be used.
A toner restricting member is brought into contact with the developing roller. Further, a toner layer thickness rusticating gap between the toner restricting member and the developing roller is narrowed.
As a result, toner is charged by the developing roller and the toner restricting member and/or frictional electrification caused by friction of toner.
However, in the developing unit of the one component non-magnetic developing system, because of centrifugal force, which is caused when the developing roller or the stirring member are rotated, impact, which is generated by friction between toner and toner thin layer forming means, or non-magnetic chain caused by non-magnetism (binding force of toner is small), toner splashes in the housing of the developing unit, and toner is leaked from the developing roller and both ends of the toner restricting member.
In order to solve these problems, a sealing member such as a teflon felt or an elastic member of urethane foam is provided on the developing roller and a side peripherary portion of both ends of the toner restricting member, thereby preventing toner from being splashed.
However, due to the long period of use of the unit, toner is introduced between fibers or into an interior of a cell, and the fibers or the interior of the cell is saturated with toner. As a result, it is known that a sealing function is considerably lowered. Therefore, in one component non-magnetic developing unit, there are problems in that means for providing a surely sealing mechanism must be prepared, a life of the sealing member used in the sealing mechanism must be considered.
FIGS. 9 and 10 are schematic views of the one component non-magnetic developing unit, seen from a direction where the developing roller is opposite to the photosensitive member.
In FIG. 9, a width W1 of a toner restricting member b is formed to be shorter than a width W2 of a developing roller a. Sealing members c serve as both side seals of toner restricting member b and the developing roller a. Also, the sealing members are provided in the side frames so as to enclose the surroundings of both ends of the developing roller a. A recovery blade e for receiving dropped toner is provided at a position corresponding to a lower stream of the toner restricting member b along the direction where the developing roller a is rotated.
In the developing unit of FIG. 9, if the width W1 of the toner restricting member b is formed to be shorter than the width W2 of the developing roller a (W1&lt;W2) and toner shown by f is leaked from the sealing members c to an area, which is shown by widths W3 of both sides of the toner restricting member b, uncharged toner, which is not frictional between toner and the toner restricting member b, is transferred from the developing roller a. In this case, since toner f' does not obtain sufficient electrical charges, which are necessary for development, toner f' is generally adhered to a non-image sections of both end portions of the photosensitive member formed to be longer than an axial length of the developing roller a. This does not have an unfavorable influence on an image to be output since the area is out of the image area. However, since non-image section of the both end portions of the photosensitive member are always developed, there are problems in that an amount of toner consumption and an amount of toner collection are increased.
FIG. 10 shows an example in which the width W1 of the toner restricting member b is formed to be longer than the width W2 of the developing roller a (W1&gt;W2).
At both end portions of the developing roller a, sealing members c, which are formed of non-rotatable members having the same diameter as the developing roller a, are formed to be sandwiched between side frames d. In FIG. 10, there is shown an example in which lubricating seals g having the same diameter as the developing roller a are provided in the respective spacer h between the developing roller a and sealing members c.
In the developing unit of FIG. 10, since the width W1 of the toner restricting member b is formed to be longer than the width W2 of the developing roller a, there can be solved the problem of the developing unit of FIG. 9, that is, a phenomenon in which uncharged toner f' is adhered to the photosensitive member.
However, it is substantially difficult to completely remove slight step difference between the developing roller a and the lubricating seals g and a slight step difference between the lubricating seals g and the sealing members c, toner f is splashed to an area shown by W4 of both end portions. Also, there is a problem in which the thickness of the toner layer becomes unequal at a pair of boundary lines, which is shown by the drawing.